The present invention relates generally to a light emitting semiconductor device and a method for fabricating the same. More specifically, the invention relates to a light emitting compound semiconductor device, which has an electrode having good adhesion and a low contact resistance, and a method for fabricating the same.
Semiconductors with the composition InxAlyGa1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, x+yxe2x89xa61), which are Group III-V nitride compound semiconductors, are direct gap semiconductors, and have a band gap varying in the range of from 1.89 to 6.2 eV by controlling x and y of the composition, so that these semiconductors are promising materials for LEDs (light emitting diodes) and semiconductor laser diodes. In particular, if the semiconductors can emit light at a high luminance in a wavelength region of blue, it is possible to or increase the storage capacity of various optical memory disks, and it is possible to realize a full color display. Therefore, blue light emitting semiconductor devices with the composition InxAlyGa1xe2x88x92xxe2x88x92yN have been rapidly developed to stabilize their characteristics and to improve their reliability. Throughout the specification, it is assumed that the compositions xe2x80x9cInxAlyGa1xe2x88x92xxe2x88x92yNxe2x80x9d include all the compositions wherein the composition ratios x and y vary in the range of from 0 to 1. For example, it is assumed that the compositions xe2x80x9cInxAlyGa1xe2x88x92xxe2x88x92yNxe2x80x9d also include GaN (x=0, y=0).
References disclosing the structures of conventional blue light emitting device of nitride semiconductors include: Jpn. J. Appl. Phys., 28(1989)p. L2112; Jpn. J. Appl. Phys., 32(1993)p. L8; and Japanese Patent Laid-Open No. 5-291621.
In light emitting devices, an electrode section for supplying driving current has a very important role for the characteristics of the light emitting devices. In nitride compound semiconductors, it is particularly important to select the structure and material of an electrode for the nitride compound semiconductors, since it is generally difficult to obtain a good ohmic contact.
For example, in the aforementioned Jpn. J. Appl. Phys., 28(1989)p. L2112, aluminum (Al) is used as an electrode material for an n-type InxAlyGa1xe2x88x92xxe2x88x92yN layer. In the aforementioned Jpn. J. Appl. Phys., 32(1993)p. L8, gold (Au) is used as the electrode material. In the aforementioned Japanese Patent Laid-Open No. 5-291621, any one of chromium (Cr), titanium (Ti) and indium (In) is used as the electrode material.
However, if aluminum, chromium, titanium or indium is used as the electrode material for the n-type InxAlyGa1xe2x88x92xxe2x88x92yN layer, there is a problem in that the contact resistance is relatively high in all cases. That is, if an electrode of aluminum is used to prepare a LED to evaluate its current/voltage characteristics, the differential resistance thereof is as high as hundreds xcexa9 at a current of 20 mA. The current density of a semiconductor laser diode must be higher than that of a LED, so that it is required to further decrease the contact area of the electrode. Therefore, in the case of the semiconductor laser, the differential resistance in the current/voltage characteristics is further increased. As a result, there is a problem in that the operating voltage of the laser rises to increase its threshold current due to heat generation and to saturate an optical output power. This problem is not caused only in the case of the electrode of aluminum, but it is also caused in the case of an electrode of chromium, titanium or indium.
There is also a problem in that the crystallinity of a semiconductor layer deteriorates if the carrier concentration in the semiconductor layer is increased to decrease the contact resistance. For example, in order to increase the carrier concentration in an n-type InxAlyGa1xe2x88x92xxe2x88x92yN layer, if impurities are doped higher than or equal to 1xc3x971019 cmxe2x88x923, the crystallinity of the layer deteriorates and the surface morphology thereof also deteriorates. In a typical blue nitride semiconductor device, an n-type semiconductor layer is used as an underlayer, and p-type semiconductor layers are sequentially epitaxial-grown thereon. Therefore, if the crystallinity of the n-type semiconductor layer serving as the underlayer deteriorates, the surface morphology thereof deteriorates, and the crystallinity of the semiconductor layers, such as an active layer, formed thereon also deteriorates, so that there is a serious problem in that it is not possible to obtain good emission characteristic.
On the other hand, there is a problem in that the above described conventional electrode structure has a relatively high contact resistance. In particular, it is not easy to provide a good ohmic contact for the p-type InxAlyGa1xe2x88x92xxe2x88x92yN layer. Therefore, the same problems as the above problems are caused by the high contact resistance in a n-side electrode as well as in a p-side electrode.
Moreover, since the conventional electrode does not have a sufficient bond strength to the InxAlyGa1xe2x88x92xxe2x88x92yN layer, there are problems in that the device resistance is easy to increase and the electrode is easy to be peeled off. Therefore, there is a problem in that the initial characteristics of the light emitting device is not only deteriorated, but the reliability thereof is also deteriorated. Moreover, if the bond strength of the electrode is not sufficient, it is difficult to achieve a so-called flip-chip mounting. Therefore, there is a problem in that it is not possible to achieve the improvement of electrical and optical characteristics and the reduction of packaging dimension, which are able to obtained by the flip-chip mounting.
All the above described problems of the prior art are caused by the fact that the electrodes of the conventional light emitting nitride device can not meet the requirements for the improvement of the bond strength and the reduction of the contact resistance while maintaining the crystallinity of the semiconductor layers.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a light emitting semiconductor device having an electrode structure, which has a low contact resistance and a sufficient bond strength to an InxAlyGa1xe2x88x92xxe2x88x92yN layer while maintaining the crystallinity of the InxAlyGa1xe2x88x92xxe2x88x92yN layer, and a method for producing the same.
In order to accomplish the aforementioned and other objects, in the first preferred embodiment of the present invention, an electrode of a metal including a Group IV or VI element is deposited on an n-type InxAlyGa1xe2x88x92xxe2x88x92yN layer to reduce the contact resistance thereto and improve the bond strength of the electrode while maintaining the crystallinity of the semiconductor layer.
In the second preferred embodiment of the present invention, after an electrode material of carbon, germanium, selenium, rhodium, tellurium, iridium, zirconium, hafnium, copper, titanium nitride, tungsten nitride, molybdenum or titanium silicide is deposited on an n-type or p-type InxAlyGa1xe2x88x92xxe2x88x92yN layer, impurities for increasing the carrier concentration in the semiconductor layer thus obtained are ion-implanted and annealed. Thus, it is possible to reduce the contact resistance and improve the bond strength of the electrode while maintaining the crystallinity of the semiconductor layer.
That is, according to one aspect of the present invention, a first light emitting semiconductor device is characterized in that a first metal layer containing at least one element component of Group IV and VI elements is deposited on a contact region of a stacked structure of n-type InxAlyGa1xe2x88x92xxe2x88x92yN layer, which comprises a plurality of stacked semiconductor layers, and the element component of at least one of Group IV and VI elements contained in the first metal layer is diffused to penetrate into the semiconductor layers, so that the carrier density in the contact region increases and the contact resistance to the first metal layer decreases.
The first metal layer preferably contains, as a major component, at least one metallic element selected from the group consisting of gold, nickel, silver, titanium and aluminum.
The element component is preferably at least one element selected from the group consisting of carbon, silicon, oxygen, sulfur and selenium.
According to another aspect of the present invention, a second light emitting semiconductor device is characterized in that a first metal layer of at least one element selected from the group consisting of carbon, germanium, selenium, rhodium, tellurium, iridium, zirconium, hafnium, copper, titanium nitride, tungsten nitride, molybdenum and titanium silicide, is deposited on a contact region of a stacked structure of InxAlyGa1xe2x88x92xxe2x88x92yN layer comprising a plurality of stacked semiconductor layers.
The contact region is preferably formed so that the ion implantation of impurities is carried out via the first metal layer to increase the carrier concentration and decrease the contact resistance to the first metal layer.
The conductive type of the contact region is preferably n type, and the impurity is preferably a Group IV or IV element.
The impurity is preferably at lease one element selected from the group consisting of carbon (C), silicon (Si), tin (Sn), sulfur (S), selenium (Se) and tellurium (Te).
Alternatively, the conductive type of the contact region may be p type, and the impurity may be a Group II element.
The Group II element is preferably any one of beryllium (Be), magnesium (Mg), zinc (Zn), calcium (Ca), strontium (Sr), cadmium (Cd) and mercury (Hg).
According to a further aspect of the present invention, there is provided a method for fabricating the first light emitting semiconductor, which comprises the steps of: forming a stacked structure by stacking a plurality of compound semiconductor layers containing at least an n-type InxAlyGa1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, x+yxe2x89xa61) layer; depositing a first metal layer containing at least one element component of Group IV and VI elements on at least a part of the n-type InxAlyGa1xe2x88x92xxe2x88x92yN layer; and raising temperature to diffuse the element component, which is contained in the first metal layer, in the n-type InxAlyGa1xe2x88x92xxe2x88x92yN layer to increase a surface carrier concentration to decrease a contact resistance to the first metal layer.
The first metal layer is preferably deposited by the sputtering method.
According to a still further object of the present invention, there is provided a method for fabricating the second light emitting semiconductor, which comprises: a step of forming a stacked structure by stacking a plurality of compound semiconductor layers containing at least an InxAlyGa1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, x+yxe2x89xa61) layer; a deposition step of depositing a first metal layer on at least a part of the InxAlyGa1xe2x88x92xxe2x88x92yN layer; an ion implantation step of injecting an impurity into the InxAlyGa1xe2x88x92xxe2x88x92yN layer via the first metal layer; and a heat treatment step of raising temperature to activate the impurity injected into the contact region to increase a surface carrier concentration in the InxAlyGa1xe2x88x92xxe2x88x92yN layer to decrease a contact resistance to the first metal layer.
In the deposition step, the thickness of the first metal layer is preferably in the range of from 1 nm to 500 nm. In the ion implantation step, the acceleration voltage is preferably in the range of from 10 keV to 1000 keV, and the dose is preferably in the range of from 1xc3x971013 ion/cm2 to 1xc3x971017 ion/cm2. In the heat treatment step, the heat treatment temperature is preferably in the range of from 400xc2x0 C. to 1200xc2x0 C.
With this construction, the present invention has the following advantageous effects.
First, according to the present invention, it is possible to reduce the contact resistance of the electrode of the light emitting semiconductor device. Therefore, it is possible to decrease the operating voltage of the light emitting semiconductor device, and it is possible to improve the emission characteristics by decreasing the optical output power caused by heat generation and suppressing the threshold current rise.
According to the present invention, it is also possible to improve the bond strength of the electrode of the light emitting semiconductor device. Therefore, it is possible to inhibit the deterioration of the reliability, such as the increase of the device resistance and the contact failure due to the peeling of the electrode. In addition, it is possible to improve the physical durability to oscillation and so forth, so that the reliability of a digital versatile disc (DVD), an optical disk regeneration unit, an optical communication system, a display and so forth, on which the light emitting semiconductor device is mounted, can be considerably improved so as to be easily handled. As a result of the improvement of the bond strength of the electrode, it is possible to easily achieve the so-called flip chip packaging. Therefore, it is possible to simplify a packaging process to improve the electrical and optical performances of the light emitting semiconductor device in the packaging state, and it is possible to reduce the dimension thereof.
According to the present invention, since the calorific value of the light emitting semiconductor device decreases, it is possible to inhibit the diffusion and penetration of the impurity of the electrode material and so forth, which cause crystal defects in the semiconductor layer of the light emitting semiconductor device. Therefore, it is possible to prevent the deterioration of emission characteristics due to DLD and so forth caused by the crystal defects, so that it is possible to extend the life time of the emission characteristics of the light emitting semiconductor device to improve the reliability thereof.
It is also possible to improve a surge voltage resistance. Therefore, it is possible to improve the reliability of the light emitting semiconductor device, and it is not required to provide any protecting means and protecting circuits, which are conventionally required for surge.
In addition, it is possible to obtain various conspicuous advantages as described above, by using a conventional fabrication equipment as it is without the need of the preparation of special raw materials.
Thus, according to the present invention, it is possible to produce a light emitting semiconductor device having a high performance and high reliability in a high yield by using a simple process, so that the present invention has considerable industrial merits.